1. Field of the Invention
The present invention relates to a method of forming a sintered body of inorganic powder, and more particularly to a method of removing glass adhered to the sintered body during hot isostatic pressing process.
2. Discussion of the Prior Art
In the manufacturing of sintered objects, a preformed body of inorganic powder coated with glass is subjected to hot isostatic pressing during which the preformed powder body is isotropically pressed with a pressure medium such as inert gas under pressure at a high sintering temperature. The hot isostatic pressing is advantageous to manufacture sintered objects of complicated shapes having the same strength in all directions at a high sintering temperature. During the hot isostatic pressing process, however, the glass used as a sealing material is inevitably adhered to the sintered powder body.
To remove the adhered glass from the sintered powder body, there have been proposed a hammering method of applying a mechanical external force to the sintered powder body, a method of crystallizing the glass adhered to the sintered power body under control of its cooling temperature after sintering to facilitate removal of the glass, and a method of vaporizing the pressure medium melted in the layer of glass for foaming the glass and of cooling the foamed glass, as disclosed in Japanese Patent Publication No. 62-22953. In European Patent No. 0320927-A-1, there has been proposed a method of treating the adhered glass of the sintered powder body with water or vapor at a high temperature.
In the hammering method, it is required mechanically to apply a great external force to the sintered powder body for removal of the glass adhered thereto. This results in an increase of the manufacturing cost of this kind of sintered body. The second method, crystallization is effective only in use of special glass such as silica glass. It is, however, impossible to remove completely the adhered glass from the sintered power body. In the third method, foaming of the glass is greatly influenced by a condition of the hot isostatic pressing and the kind of the glass. It is, therefore, very difficult fully to foam the adhered glass of the sintered powder body. Even if the first hammering method was adapted prior to or after the third vaporizing method, it would be very difficult to remove completely the glass adhered to recessed portions of the sintered powder body.
In actual practices of the fourth method, the adhered glass may not be removed only by the water or vapor at a high temperature. It is, therefore, suggested in the specification of the European patent to use sodium hydroxide or other alkali compound for the fourth method. However, there is not disclosed any specific suggestion for use of the alkali compound, particularly in relation to a removal condition of the glass adhered to the sintered powder body.